Democratic opponents of GOP efforts to overhaul the tax code cited a new report Friday as evidence that such tax rewrites – combining reductions of tax breaks with lowering of tax rates – can’t produce much in the way of rate relief.

But Republicans immediately assailed the study by the nonpartisan Joint Committee on Taxation as incomplete, and said far more can be accomplished than its preliminary findings suggest.

The new JCT study looked at the impact of repealing several popular tax breaks, including all itemized deductions; taxing capital gains and dividends as ordinary income; and repealing the tax-free status of state and local bonds. It also assumed that the overhaul would include repealing the alternative minimum tax, a tax originally intended to hit wealthy people that now affects millions of middle-class households.

The JCT calculations suggested that after all those changes, income-tax rates could only be reduced by 4%. Mitt Romney has proposed reducing tax rates by 20%.

The JCT didn’t include the impact of repealing some very large tax breaks, including the exclusion for employer-provided health care. Also, the study assumes that all the Bush tax cuts and other current breaks expire, generating a huge $4.5 trillion tax increase over the next decade. However, the JCT assumes only about $700 billion goes to rate reduction, according to a memo by a budget watchdog group, the Committee for a Responsible Federal Budget.

The study also appears to run counter to some previous findings by experts. For instance, a 2008 study by Tax Policy Center experts concluded that “eliminating tax expenditures could be used to pay for an across-the-board 44% reduction in all marginal tax rates.” That meant that the top rate could be cut to about 19.6%, although lower-income people would see an effective tax increase because of the loss of deductions.

Still, the new JCT calculations provided ammunition on Friday for Democratic opponents of overhauling the tax system, including Sen. Charles Schumer (D., N.Y.). They have become increasingly critical of tax rewrites in recent days, even as President Obama has attacked Mr. Romney’s tax plan as a boon to the rich that would require tax increases on the middle class. Mr. Schumer called this week for Congress to “scrap” the traditional approach to overhauling the tax system, which was last employed by lawmakers in 1986, saying it doesn’t work at a time of high deficits and large income gaps between rich and poor.

“This confirms that the math behind Reagan-style tax reform doesn’t work even if you completely eliminate the most popular of deductions,” Mr. Schumer said in a statement on Friday. “Broadening the base as Republicans propose would not only fail to cut the deficit, it also fails to reduce rates much. What it does do is sock middle class families with an immediate tax hike.”

Aides to Sen. Orrin Hatch (R., Utah), the top Republican on the Finance Committee, emphasized that the report did not analyze Mr. Romney’s plan or any other plan. It also did not include all the possible tax breaks that could be tightened to pay for a tax overhaul, the aides said. “If more were included, those base-broadeners would provide the means for further rate reductions and other structural reforms of the tax system,” the aides said in a statement.

Romney campaign spokeswoman Andrea Saul said in a statement: “This self-described `experiment’ says nothing about the pro-growth tax reform proposed by Mitt Romney. It has different assumptions and different revenue goals….What’s beyond dispute is that President Obama will raise taxes in his second term, which is the last thing our economy needs.”

The co-chairs of two recent blue-ribbon budget panels issued an unusual joint statement late Friday, supporting the idea of a base-broadening tax overhaul to spur growth and also reduce federal deficits.

The statement by budget expert Alice Rivlin and former Sen. Pete Domenici (R., N.M.) as well as former Sen. Alan Simpson (R., Wyo.) and former White House chief of staff Erskine Bowles said that “nothing in the JCT analysis changes our belief that it is possible for tax reform to reduce rates and produce additional revenues if policymakers are willing to make the tough choices to eliminate or scale back tax expenditures.”

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